The invention relates generally to vehicle horn and control function switch apparatus and, more particularly, to a vehicle horn and control function switch apparatus for steering wheel applications with compensation for effects of temperature and pre-loading. The invention also relates to a method of operation for the compensated switch apparatus.
From the advent of motor vehicles, there was a recognized need to gain the attention of people and animals that were in the path of a vehicle. As a result, the horn became a standard safety feature of motor vehicles, and was most swiftly and conveniently operated by a mechanical switch positioned in the center of the vehicle steering wheel. In more recent times, the airbag became another standard safety feature of motor vehicles. The most convenient location for the driver side airbag was the center of the vehicle steering wheel, collocated with the horn switch and other vehicle controls. In order to accommodate this addition, automotive designers turned to very thin electronic switch devices for horn actuation that could easily be positioned over an airbag cover. However, these devices suffered from the effects of temperature changes and static forces from adjacent components resulting from the manufacturing process or material aging characteristics.
These very thin electronic switch devices are usually fabricated from membrane material, piezoelectric elements, or pressure sensitive resistive ink, and require supporting electronic circuitry in order to function. U.S. Pat. Nos. 5,463,258 and 5,539,259 disclose identical circuits for sensing a resistive change in a force transducer due to applied forces, for turning on a transistor switch, for energizing a relay, and for activating a horn. Neither patent discloses any means of temperature or preload force compensation.
For reliable operation over extreme temperature ranges encountered in vehicle applications, compensation schemes are required for the very thin electronic switch devices. One such compensation scheme is described in U.S. Pat. No. 5,576,684. This scheme makes use of a flexible potentiometer that changes resistance as its shape changes as the user presses against the steering wheel hub cover. The horn control circuit responds to extremely rapid changes in the resistance of the flexible potentiometer, but not to more gradual changes caused by, for example, temperature changes. An analog to digital converter and a microprocessor are the primary circuit control components. Another similar approach is described in U.S. Pat. No. 5,398,962. The apparatus comprises a force sensor fabricated from pressure sensitive resistive material and disposed in a cover assembly mounted on the steering wheel. The sensor is connected to a circuit for sensing the change in applied force to the sensor during a predetermined wait period, for determining whether the change has reached a selected threshold value, and for activating or deactivating a horn if the change at least equals the selected threshold value. Yet another approach is disclosed in U.S. Pat. No. 5,489,806, which discloses a pressure or deflection sensitive horn switch with temperature compensation to compensate for changes in the plastic airbag cover stiffness due to changes in temperature. The sensors comprise both a temperature sensing device and a horn switch deflection sensing device operated by applied force mounted on the airbag cover surface and connected to an analog to digital converter controlled by a microprocessor. The microprocessor evaluates and compares the temperature and deflection signals to determine if the deflection of the cover surface is sufficient, when compensating for change in cover stiffness due to temperature change of the cover surface, to actuate a horn. Although these approaches provide for some temperature variation, it is not apparent that the wide sensor conductance variations with temperature of these sensing devices can be accommodated. There is also no disclosure of compensation for variations in preload forces due to manufacturing processes or material aging. Another related problem not addressed is the ability to maintain the maximum dynamic range of the force sensor relative to the fixed operating range of the supporting circuitry for all operating temperatures and preload conditions.
For reliable operation over extreme temperature ranges encountered in vehicle applications, there is a need for a horn and control function switch temperature compensation means that enables reliable operation over a temperature range from about -40.degree. C. to about +85.degree. C. In order to account for changes in preload forces resulting from the manufacturing process or vehicle component material aging, a horn and control function switch preload compensation means must enable reliable operation over a range of applied force to the switch sensor of from 1 Kg up to 5 Kg. Since, upon an applied force to the switch sensor of from 1 Kg up to 5 Kg, a typical resistance range of a horn and control function switch sensor is from 39 Kohms to 188 Kohms at -40.degree. C., and from 2 Kohms to 12 Kohms at 85.degree. C., there is a need to match this extremely wide dynamic sensor operating range to the fixed operating range of the control circuitry with suitable dynamic range compensation means. In addition to these requirements, the horn and control function switch, and associated circuitry must be easily and inexpensively fabricated, assembled, tested, calibrated, and installed.